1. Field of the Invention
The invention relates to an image forming apparatus of the electrophotographic type such as a copying machine or a laser beam printer, or an image forming apparatus of the electrostatic recording type, and a driving apparatus for driving image forming means used in the image forming apparatus.
2. Related Background Art
As one of color image forming apparatuses, there is known a color laser beam printer of the tandem type, in which a plurality of photosensitive drums are juxtaposed, for effecting the formation of an image by superimposing toners of four colors, i.e., yellow, magenta, cyan and black, one upon another. Driving the photosensitive drums which are image bearing members highly accurately is an important matter to obtain an image of good quality suffering little from banding (an uneven tone of an image) and color misregister.
FIG. 8 of the accompanying drawings shows an example of a conventional driving construction for a photosensitive drum. The reference numerals 21 and 22 designate the front side plate and rear side plate (front and rear frame metal plates), respectively, of the main body of an image forming apparatus. A drum shaft 71 is rotatably bearing-supported between the front and rear side plates 21 and 22 through bearings 23 and 24. The photosensitive drum 11 is fixedly supported on the drum shaft 71.
The frame 32 of a drum driving unit is mounted on and fixed to the outside of the rear side plate 22. The rear end portion of the drum shaft 71 is rearwardly extended and is bearing-supported on the frame 32 through a bearing 33 and extends through the frame 32 to thereby protrude outwardly of the frame.
The reference numeral 34 denotes a motor mounted on and fixed to the inner side of the frame 32, and the reference numeral 35 designates a motor gear secured to the rotary shaft of this motor 34. A drum shaft gear 51 integrally disposed on the rear end portion of the drum shaft 71 is in meshing engagement with the motor gear 35. The motor gear 35 is a small-diametered gear and the drum shaft gear 51 is a large-diametered gear, and these gears together constitute a reduction gear train. The drum shaft gear 51 is fixed to the drum shaft 71 by the use of a spring pin 52, a compression spring 53 and a locating snap ring 54. The spring pin 52 is a circumferential coupling member for the drum shaft gear 51 and the drum shaft 71, and effects the transmission of drive. The compression spring 53 which is a biasing member and the locating snap ring 54 effect the axial positioning of the drum shaft gear 51 relative to the drum shaft 71. This construction is a construction in which the rotatively driving force of the motor 34 is transmitted to the drum shaft 71 through the motor gear 35 and the drum shaft gear 51 which are the reduction gear train to thereby rotatively drive the drum shaft 71, i.e., the photosensitive drum 11.
The uneven rotation of the photosensitive drums may be mentioned as one of the causes of the occurrence of banding and color mitsregister. The uneven rotation of the photosensitive drums is caused by the speed fluctuation of the meshing frequency of the gear train of the drum driving system and besides, may be caused by the transmission of the speed fluctuation of image forming process means acting on the photosensitive drums, particularly the speed fluctuation of recording material transporting means for transferring toner images to a recording material and transporting it or the speed fluctuation of an intermediate transfer member.
As an ordinary method of preventing banding, there is known a method of providing a flywheel 61 on the drum shaft 71, as shown in FIG. 8. This intends to contrive the suppression of the speed fluctuation of the photosensitive drums in an area of a relatively high frequency zone by an inertial effect. The flywheel 61 is fixed to the drum shaft 71 by the use of a screw 62.
On the other hand, as a method of preventing color misregister in an apparatus having a plurality of photosensitive drums, there are known the following two ways of view:
(1) The speed fluctuation of one revolution period of the photosensitive drum is made small to thereby reduce the expansion and contraction of an image of each color, and as a result, color misregister is suppressed.
(2) The amplitudes and phases of the expansion and contraction of images of respective colors caused by the speed fluctuation of one revolution period of the photosensitive drum are tuned to thereby cancel color misregister.
When an attempt is made to achieve an improvement in the accuracy of rotation of the photosensitive drum 11 along the way of view mentioned under item (1) above by the driving construction of FIG. 8, it is most important to make the vibration of the drum shaft gear 51 which is a driven gear provided on the drum shaft 71 small.
This will now be described. When the meshing position of two gears fluctuates in the radial direction thereof, the fluctuation of the rotating speed of the driven side occurs in proportion to the amount thereof. This is because the radial fluctuation of one revolution period, i.e., the vibration, of the driven gear is directly geometrically converted into the speed fluctuation of the one revolution period.
Here, the vibration of the above-described gear refers not to the vibration of the gear singly, but to the vibration in a state in which it is coupled to the drum shaft. To make the vibration of the gear in its coupled state small, it is necessary to make the vibration of the gear singly (a whole pitch meshing error) small and also, to couple the gear to the drum shaft so that there may be no eccentricity or inclination.
In the aforedescribed example of the conventional art, however, there are the following problems. Description will first be made of the problem of the flywheel provided with a view to prevent banding. In an apparatus of the tandem type, the radius of the flywheel is restricted by the interval between adjacent drums. Therefore, in order to secure a necessary amount of inertia, such design as increasing the thickness of the flywheel or axially offsetting and disposing the flywheel is usually made, but this results in the bulkiness of the apparatus.
Also, strength and rigidity are required of a drum shaft, a bearing, a frame, etc. for supporting the flywheel, and this results in a disadvantage in costs. Also, the addition of the flywheel leads to a corresponding increase in the necessary starting torque of a motor and therefore, a larger motor becomes necessary, and this means disadvantages in space and costs.
Further, in the case of a specification wherein the process speed of image formation is slowed down in conformity with the kind of the transfer material such as thick paper or OHT, there is the disadvantage that the inertial effect of the flywheel is greatly decreased. This is due to the fact that the inertial effect is proportional to the square of the speed, and when for example, the process speed is a half speed, the inertial effect becomes xc2xc, and when the process speed is a quarter speed, the inertial effect becomes {fraction (1/16)}. As a result, the flywheel is effective at {fraction (1/1)} speed (standard speed), but in a low speed mode, there is a case where the effect of the prevention of banding is not obtained. If the amount of inertial is determined in accordance with a low speed, the above-described various disadvantages will greatly increase.
Also, a scissors gear often used in the driving system of an automobile can effect smooth driving because a main gear and a sub-gear are mutually rotatably biased and backlash is eliminated, and can be expected to be effective to prevent banding, but is difficult to assemble and is not popular in business machines including image forming apparatuses.
Description will now be made of a problem peculiar to a method of coupling the drum shaft and the gear together in the aforedescribed example of the conventional art regarding color misregister.
The necessity of coupling the gear to the drum shaft so that there may be no eccentricity is apparent from the foregoing description that the fluctuation of the rotating speed occurs from the radial fluctuation of the meshing position, and description will now be made of the reason for the necessity of the gear being fixed to the drum shaft so that there may be no inclination.
A helical gear is usually used as the gear of a drum driving system. This is because the helical gear is higher in contact ratio than a spur gear and is small in the speed fluctuation of a meshing frequency and is very effective for the prevention of banding. However, the helical gear has a helix angle and therefore has the demerit that when the meshing position of the gear axially fluctuates, it leads to a rotation error. Therefore, it becomes necessary to fix the gear to the drum shaft so that there may be no inclination.
It is necessary that the coupling of the gear and the drum shaft be done so that there may be no lash in the circumferential direction. This is because the presence of lash in the circumferential direction would cause a speed fluctuation if the load fluctuation of the photosensitive drum occurs under the influence of disturbance such as sheet feeding, and would result in color misregister or shock images.
The example of the conventional art shown in FIG. 8 is a construction in which the transmission of drive is effected with a spring pin 52 interposed between the drum shaft 71 and the drum shaft gear 51. The spring pin 52 is forced into a hole formed in the drum shaft 71 and on the other hand, is lightly forced into a groove portion formed in the drum shaft gear 51 and as a result, the drum shaft gear 51 and the drum shaft 71 are fixed to each other without any lash in the circumferential direction. Also, the dimensional relationship between the inner diameter of the drum shaft gear 51 and the outer diameter of the drum shaft 71 is made such that the gap therebetween is small to the utmost so as to decrease the eccentricity during the coupling thereof.
In the above-described construction, however, misalignment in centering occurs without fail between the drum shaft 71 and the spring pin 52 and between the spring pin 52 and the drum shaft gear 51 and as a result, the drum shaft gear 51 is fixed to the drum shaft 71 in its eccentric state. This occurs in a construction wherein the spring pin 52 extends through the drum shaft 71 and is engaged with the drum shaft gear 51 at two locations in the circumferential direction, as well as in a construction wherein the spring pin 52 does not extend through the drum shaft 71 to thereby form a one-arm construction and is engaged with the drum shaft gear 51 at one location in the circumferential direction, although there is a difference in degree.
Further, the amount of eccentricity of the drum shaft gear 51 relative to the above-described drum shaft 71 is not constant in the width wise direction (axial direction) of the gear and thus, causes also the inclination of the drum shaft gear 51 relative to the drum shaft 71. Accordingly, the above-described construction, even when the accuracy of the gear singly is good, results in the uneven rotation of the photosensitive drum 11 attributable to the coupling method for the drum shaft gear 51 to the drum shaft 71, and as a result, leads to color misregister.
Besides the above-described construction, as the coupling method for the gear, there is known a method of fastening a set screw or the like in a direction orthogonal to the axis of the gear to thereby couple the gear to the drum shaft, but in this method, the inclination of the gear becomes greater than in the aforedescribed fixing method using the spring pin, and this method is most undesirable.
The present invention has been made in view of the above-noted problems and the object thereof is to provide an image forming apparatus which contrives the prevention of banding during image formation and which is also excellent in assembling property, and a driving apparatus for image forming means used in the image forming apparatus.
A driving apparatus for image forming means used in an image forming apparatus for achieving the above object comprises:
a drive source for generating a driving force;
a drive gear for transmitting the driving force from the drive source;
a drive shaft for transmitting the driving force to the image forming means;
a first gear and a second gear fitted to the drive shaft and meshing with the drive gear;
biasing means engaged with the first gear and the second gear and capable of circumferentially biasing the two; and
changeover means capable of changing over the biasing means between into a biasing state and into a bias-releasing state in a state in which the first gear and the second gear are in meshing engagement with the drive gear.
Also, an image forming apparatus for achieving the above object comprises:
image forming means for forming an image;
a drive source for generating a driving force for driving the image forming means;
a drive gear for transmitting the driving force from the drive source;
a drive shaft for transmitting the driving force to the image forming means;
a first gear and a second gear fitted to the drive shaft and meshing with the drive gear;
biasing means engaged with the first gear and the second gear and capable of circumferentially biasing the two; and
changeover means capable of changing over the biasing means between into a biasing state and into a bias-releasing state in a state in which the first gear and the second gear are in meshing engagement with the drive gear.